Study: Warming Lake Superior affecting fish

Newly published research has found that Lake Superior's warming water probably is affecting its most abundant big fish. Increasing water temperatures over the last three decades have made conditions more favorable for chinook salmon, walleyes and...

Chinook salmon
(1998 file / News Tribune)

Newly published research has found that Lake Superior's warming water probably is affecting its most abundant big fish.

Increasing water temperatures over the last three decades have made conditions more favorable for chinook salmon, walleyes and lean lake trout but less favorable for cold-water-loving siscowet lake trout.

The study, using a mix of computer modeling and temperature measurements, estimates that fatty siscowets have lost about 20 percent of their historic habitat because of the temperature changes that already have occurred.

The research was conducted by the University of Wisconsin-Madison, funded by Wisconsin Sea Grant.

The research builds on work by Jay Austin and other researchers at the University of Minnesota Duluth's Large Lakes Observatory, who found that average Lake Superior surface water temperatures increased 2.5 degrees Celsius between 1979 and 2006, among the most dramatic examples of climate change in North America.


Since then, the rapid warming phenomenon has been found in other big lakes, including Lake Baikal in Russia. And ongoing research by the UMD crew appears to show it continuing since 2006; in August 2010, the average temperature taken at three Lake Superior buoys was the highest in the 31 years of records.

"I think our biggest message is that these are changes that already have happened. These are not projections of temperatures years from now," said Tim Cline, lead author of the published report who is now at the School of Aquatic and Fishery Sciences at the University of Washington. "We think Lake Superior deserves attention because these are some of the biggest temperature changes we've seen anywhere. We wanted to see how that may be already affecting the fish."

The researchers picked lake trout, siscowet, salmon and walleye because they are among the most important species for sport angling and the region's tourism economy. They used a "three-dimensional hydrodynamic computer model" developed by co-author Val Bennington, based at UW-Madison, to map changes in Lake Superior water temperatures. Then they mapped those temperatures to those preferred by the four fish species.

Siscowet prefer temperature about 4 degrees Celsius, just over 39 degrees Fahrenheit, while lean lake trout, the more sought-after species by anglers, prefer about 50 degrees Fahrenheit.

"We found that the number of days with preferred temperatures and the amount of water available within the preferred temperature range has increased significantly for lean lake trout, salmon and walleye," Cline told the News Tribune. Meanwhile, higher water temperatures have forced siscowets to move farther from shore.

The research also noted that the eastern portion of the lake is warming faster than the west.

"That's probably one of the most surprising things to me is that the eastern end of the lake is warming much more rapidly, especially in the more recent years," Cline said. That's likely because of prevailing westerly winds blowing sun-warmed water to the east, and because underwater currents bring more cool water toward the west end, he said.

Between 1979 and 2006, salmon and walleyes gained seven days in each decade that were in their preferred temperature range, while lean lake trout gained six. But siscowet trout lost three days each decade on average, with the fastest loss coming in the last few years. Cline said. By 2006, siscowets had 15 fewer days per year in their preferred temperatures range. The decline likely has continued since then.


The fish also gained or lost range, or habitat, based on those changes. Lean lake trout and salmon gained about 11,583 square miles of habitat in their preferred temperature zone while walleyes gained a whopping 19,305 square miles. But siscowet trout lost about 3,861 square miles, or nearly 20 percent, of their historical habitat.

Cline cautioned, however, that the decline in water with preferred temperatures doesn't necessarily mean the number of fish has changed correspondingly.

"We don't know if there are that many fewer siscowet and more walleye," Cline said. "But we think the lake already is seeing changes in the makeup and distribution of fish."

In the past, cold water in the lake has favored siscowets over lean lake trout, with the fatty fish outnumbering the leaner ones 5-1. But that relationship could change as the lake warms, said James Kitchell, a retired UW-Madison professor who has studied the effect of warmer water on sea lamprey, a fish parasite. Now, the temperature data predict a more even ratio between the two trout species.

John Myers reports on the outdoors, natural resources and the environment for the Duluth News Tribune. You can reach him at
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